I very rarely get a blue primary engram. I always get chest pieces and helms. Out of using 30 three of coins I've gotten 6 crest of alpha lupi and 2 mask of the third man helms. I use one at the beginning of heroic strikes
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Dont use them at the beginning because most strikes have a middle mini boss that is a major or ultra. Wait til you are about to fight the final boss and use one or two. Works pretty well for me and my friends. Usually get one exotic every 3 to 5 strikes but all i ever get are armors. Ive had to buy my exotic weapons